Dynamics Studies Of The Flame Front Deformation Instability In The Porous Media Premixed Combustion

As a new clean burning mechanism,the technology of porous media premixed combustion has a lot of advantages such as the higher combustion efficiency,the less pollution,the looser limit of lean flammability and the more flexible load regulation,which can be not only suitable for our energy structure adjustment of coal to nature gas,but also have great significances on the utilization of the low heating-value gas and the reduction of the pollution.However,the flame front deformation instabilities such as inclining,splitting,shifting during the process of the premixed combustion in the porous media seriously impact the development of the industrial application and attract researchers' attention.Different from the assumption that the flame front has an initial inclined angle in many reports,this article insists that the initial inclined angle could be resulted by the inhomogeneous preheating temperature distribution in the preheating zone.For this reason,the effects of the initial preheating temperature difference on the flame front inclining,splitting and shifting instabilities in porous media premixed combustion are verified and the dynamics mechanisms are discussed experimentally and numerically.The main research contents and results are as follows:1.The experiments.The experimental facilities are proposed,in order to investigate the flame front deformation instabilities in the methane/air premixed combustion within porous media.The effects of the initial preheating temperature difference on the flame front inclining and splitting instabilities are verified,and the effects regimes under different equivalence ratios and inlet gas velocities are discussed.The results indicate that the inhomogeneous preheating temperature distribution in the initial preheating zone is indeed one motivation of the flame front inclining,splitting instabilities in the porous media premixed combustion.With the preheating temperature difference,the smaller equivalence ratio and the larger inlet gas velocity can result in the more serious deformation of the flame front.2.The numerical simulations.The two-dimensional,two-temperature model of the methane/air premixed combustion in the porous media is presented.The fluid flow,heat transfer and combustion are calculated by the software FLUENT 6.3,in order to investigate the evolution of the flame front deformation instabilities.The variations of the shape and location of the flame front are comparatively researched under various condition parameters of the initial preheating temperature differences,equivalence ratios,inlet gas velocities and the length of burner.Simultaneously,the evolution regularities and the dynamics mechanisms of the flame front deformation instabilities are analyzed in detail.The conclusions are arrived as follow:(1)Inclining of the flame front.The inhomogeneous preheating temperature distribution in the initial preheating zone is indeed one motivation of the flame front inclining instability in the porous media premixed combustion.It can be observed that the larger the initial preheating temperature difference is,the faster the flame front inclined angle increases and the more intensively the inclining instability grows up.The process of the flame front evolution can be divided into three stages in which the inclined angle grows slowly,speedily,and tends to a constant in the last stage.With the same initial preheating temperature difference,the flame front inclining instability is stronger,the inclined angle grows faster and the final inclined angle is bigger,when it is under a smaller equivalence ratio and a larger inlet gas velocity.The numerical results are basically in accord with the experiments,while the effect regularities of the equivalence ratio and inlet gas velocity on the flame front inclining instability are essentially consistent with the literature,which manifests the numerical results are credible.(2)Splitting of the flame front.The splitting is more serious and severe than the inclining of the flame front as an unsteady phenomenon.With the condition of the larger initial preheating temperature difference,the smaller equivalence ratio and the quicker inlet gas velocity,the flame front splitting can be achieved more easily during the propagation downstream.(3)Shifting of the flame front.When enhancing the initial preheating temperature difference,reducing the equivalence ratio or increasing the inlet gas velocity,the flame front may shift to one side of the burner,and the combustible gases may exhaust before burning,which caused the waste and pollution.The flame front shifting is an extreme phenomenon in the porous media premixed combustion.(4)The length of the burner may also influence the flame front deformation evolution in the porous media premixed combustion.When the burner is longer,the flame front propagates more stably.